Bryozoans are Major Modern Builders of South Atlantic Oddly Shaped Reefs

Alex C Bastos,Laura S Vieira,Gilberto M Amado-Filho,Jody M Webster,Ulises R Magdalena,Laís V Ramalho,Juan Carlos Braga,Rodrigo L Moura,Fernando C Moraes

doi:10.1038/s41598-018-27961-6

Abstract

In major modern reef regions, either in the Indo-Pacific or the Caribbean, scleractinian corals are described as the main reef framework builders, often associated with crustose coralline algae. We used underwater cores to investigate Late Holocene reef growth and characterise the main framework builders in the Abrolhos Shelf, the largest and richest modern tropical reef complex in the South Western Atlantic, a scientifically underexplored reef province. Rather than a typical coralgal reef, our results show a complex framework building system dominated by bryozoans. Bryozoans were major components in all cores and age intervals (2,000 yrs BP), accounting for up to 44% of the reef framework, while crustose coralline algae and coral accounted for less than 28 and 23%, respectively. Reef accretion rates varied from 2.7 to 0.9 mm yr−1, which are similar to typical coralgal reefs. Bryozoan functional groups encompassed 20 taxa and Celleporaria atlantica (Busk, 1884) dominated the framework at all cores. While the prevalent mesotrophic conditions may have driven suspension-feeders’ dominance over photoautotrophs and mixotrophs, we propose that a combination of historical factors with the low storm-disturbance regime of the tropical South Atlantic also contributed to the region’s low diversity, and underlies the unique mushroom shape of the Abrolhos pinnacles.

Highlights

In the tropical and subtropical portions of the Indo-Pacific and the Caribbean it is well established that reef frameworks are built by species-rich assemblages of scleractinian corals and coralline algae[6,7]
We examine the Late Holocene (~2000 yrs BP) framework of reef pinnacles with tops at 4, 15, and 25 m water depth in the Abrolhos Shelf, based on samples obtained from rotary drilling within the mid-shelf arc
We compiled meteoceanographic historical data in order to support our interpretation of reef morphology, biodiversity and benthic assemblage structure, within the context of the low-disturbance regime of South-western Atlantic (SWA) reefs

Summary

Introduction

In the tropical and subtropical portions of the Indo-Pacific and the Caribbean it is well established that reef frameworks are built by species-rich assemblages of scleractinian corals and coralline algae[6,7]. The unusual mushroom-shaped morphology of the Abrolhos reefs and the low coral richness of the SWA was noted in the 19th Century[12,13], but framework building within this entire region remains poorly understood. We examine the Late Holocene (~2000 yrs BP) framework of reef pinnacles with tops at 4, 15, and 25 m water depth in the Abrolhos Shelf, based on samples obtained from rotary drilling within the mid-shelf arc. We analyzed accretion rates and reef-builder assemblages, and compared our data with available information from Abrolhos near-shore reefs[16] and elsewhere, within and outside the tropical SWA. We compiled meteoceanographic historical data in order to support our interpretation of reef morphology, biodiversity and benthic assemblage structure, within the context of the low-disturbance regime of SWA reefs

Methods

Results

Conclusion